1. Field of the Invention
The present invention relates to a light beam scanning device, and more particularly, to a light beam scanning device with which a photosensitive material is scanned using light beams emitted from microarea light-emitting diodes.
2. Description of the Related Art
Conventionally, when a light beam scanning device is used to expose a color photosensitive material having a spectral sensitivity in a visible region (400 to 690 nm), a color image is recorded by scan-exposing the color photosensitive material with light beams of three colors, i.e., red, green, and blue, which are modulated on the basis of image data for respective colors. Generally, single-mode semiconductor lasers are used as light sources for emitting light beams.
However, oscillation wavelength of semiconductor lasers varies depending on operation temperature and injection current, and semiconductor lasers have a mode-hopping property, that is, oscillation wavelength thereof largely varies discontinuously. This mode-hopping causes variations in light output power and wavelength, whereby image quality deteriorates.
Semiconductor lasers that are currently on the market include GaN type semiconductor lasers having oscillation wavelengths around 410 nm, AlGaInP type semiconductor lasers having oscillation wavelengths of 630 to 680 nm, and AlGaAs or GaInAsP type semiconductor lasers having oscillation wavelengths of 780 to 1550 nm. Since the only semiconductor lasers that are available are those having oscillation wavelengths in these predetermined ranges, a color photosensitive material having a spectral sensitivity in the visible region cannot be scan-exposed using a light beam having a desired wavelength. Particularly, semiconductor lasers emitting green and blue light beams having oscillation wavelengths of 450 to 550 nm for exposing silver halide photosensitive materials having spectral sensitivities in a wavelength range from 450 to 550 nm have not yet been put into practical use.
A variety of microarea light-emitting diodes emitting light in various wavelength ranges are available. For example, edge emitting light emitting diodes emitting green and blue light beams are disclosed in Japanese Patent Application Laid-Open No. 2000-332291 (Nichia Corporation). Therefore, the above problem can be solved by using such microarea light-emitting diodes. However, with microarea light-emitting diodes, the amount that the wavelength varies with respect to changes in applied current is larger in comparison with ordinary semiconductor lasers. Therefore, when an applied current is changed to change image density, not only intensity but also wavelength of a light beam is changed. Thus, quality of the image formed by the light beam whose intensity as well as wavelength are changed deteriorates.
The above-described problem which concerns variations in wavelength can be solved by using a fixed electric current for driving microarea light-emitting diodes, and modulating pulse widths of the microarea light-emitting diodes according to tone information of image data so that changes in the applied current do not directly affect variations in wavelength.
However, if the pulse widths are modulated according to image data, the microarea light-emitting diodes may be continuously lit depending on tone information. If the microarea light-emitting diodes are kept lit for a long period, properties thereof, such as light output power, wavelength, and the like, change due to heat generated by the microarea light-emitting diodes.
In order to solve the above-described problem, an object of the present invention is to provide a light beam scanning device in which microarea light emission diodes which are free from variations in light output power and wavelength due to mode-hopping are used to prevent changes in properties such as light output power and the like due to heat generation caused by applied current.
In order to accomplish the above-described object, a first aspect of the present invention includes a light source provided with a microarea light-emitting diode having microarea light-emitting regions; a modulator for generating a pulse signal including at least one pulse having a period shorter than a period for forming an image corresponding to one pixel on the basis of image data, and modulating light beams emitted from the microarea light-emitting diode with the pulse signal; and a scanner for scanning a photosensitive material with the modulated light beams.
According to the first aspect of the present invention, since the microarea light-emitting diodes having a microarea light-emitting region are used, no variation in light output power and wavelength due to mode-hopping is caused. The light beams emitted from the microarea light-emitting diodes are modulated by the pulse signal including at least one pulse having a period shorter than a period for forming an image corresponding to one pixel to make periods of the pulses shorter than the period for forming an image corresponding to one pixel. Therefore, a pause is generated between the pulses of the pulse signal, so that the microarea light-emitting diodes are not continuously driven over several pixels and light emission from the microarea light-emitting diodes are stopped during the pause to prevent changes in properties due to heat generation.
In the first aspect of the present invention, since changes in properties due to heat generation can be prevented by generating the pause between adjacent pulses, the period of the pulses may be modulated within the period for forming an image corresponding to one pixel according to image data, or, as described in a second aspect of the present invention next, pulses having a constant period and a substantially constant power may be used, and a number of the pulses within the period for forming an image corresponding to one pixel may be determined on the basis of image data.
A second aspect of the present invention includes a light source provided with a microarea light-emitting diode having microarea light-emitting regions; a modulator for determining a number of pulses having a constant period and a substantially constant power within a period for forming an image corresponding to one pixel on the basis of image data, and modulating light beams emitted from the microarea light-emitting diode with a pulse signal including the pulses; and a scanner for scanning a photosensitive material with the modulated light beams.
The second aspect of the present invention uses the microarea light-emitting diodes, and the light beams emitted from the microarea light-emitting diodes are modulated with the pulse signal including the pulses having a constant period and a substantially constant power. Therefore, a pixel is scanned by at least one light beam having a constant beam spot and a substantially constant light output, and a density of a pulse-shaped current within a constant period is changed to record the pixel. Therefore, similarly to the first aspect of the present invention, a pause is generated between the pulses of the pulse signal, and the microarea light-emitting diodes are not continuously driven over several pixels. Thus, changes in properties of the microarea light-emitting diodes due to heat generation can be prevented.
The period of the pulse can be less than one tenth of the period for forming an image corresponding to one pixel. Therefore, one pixel is exposed with ten or more light beams, and a high quality image can be formed.
By forming an image corresponding to one pixel by repeating scanning with the light beam modulated by the pulse signal in a main-scanning direction several times in a sub-scanning direction, the pixel is scanned by several scan lines in the sub-scanning direction. Thus, an image corresponding to one pixel is formed by being exposed several times respectively in the main-scanning direction and in the sub-scanning direction, and an image having high quality also in the sub-scanning direction can be formed.
The number of the pulses forming an image corresponding to one pixel can be determined on the basis of tone information obtained from image data.
The light source can include a microarea light-emitting diode emitting a light beam in a wavelength range corresponding to blue, a microarea light-emitting diode emitting a light beam in a wavelength range corresponding to green, and a microarea light-emitting diode emitting a light beam in a wavelength range corresponding to red. Thus, a color photosensitive material can be scan-exposed to form a color image.
Microarea light-emitting diodes having an area of a light-emitting region from 0.1 xcexcm2 to 64 xcexcm2 can be used as the microarea light-emitting diodes described above. Light beams emitted from the microarea light-emitting diodes are incoherent light, and do not have strong directivity like a laser beam. However, an amount of light necessary for scan-exposure can be ensured. In addition, a microarea light-emitting diode can be used in a wider wavelength range than a semiconductor laser.
The light source of the present invention may include microarea light-emitting diodes, and a microarea light-emitting diode emitting a light beam of a wavelength range corresponding to blue, a wavelength-conversion solid laser or a second-harmonic generation laser emitting a light beam of a wavelength range corresponding to green, and a semiconductor laser emitting a light beam of a wavelength range corresponding to red can be used in combination. Alternatively, a microarea light-emitting diode emitting a light beam of a wavelength range corresponding to blue, a microarea light-emitting diode emitting a light beam of a wavelength range corresponding to green, and a semiconductor laser emitting a light beam of a wavelength range corresponding to red may be used in combination.